Toad and frog urinary bladder have served for a long time as model systems in which to study the basic physiologic principles of antidiuretic hormone (ADH)-induced water permeability. There is considerable evidence to suggest that this increased permeability is localized heterogeneously amongst granular cells, the predominant cell type in these tissues. There is also microelectrode evidence demonstrating that these cells are electrically coupled. The specific aims of this proposal are 1) to identify directly which cell types respond to ADH (i.e. granular and/or mitochrondria-rich), 2) demonstrate directly the presence of a heterogeneous distribution of ADH-induced water permeability amongst granular cells and 3) investigate whether or not intercellular communication plays a role in the hormone response, by microinjecting concurrently a fluorescent dye and cAMP, the latter a known intracellular mediator of the response, into an individual granular cell and measuring the response of the surrounding granular cells. These experiments will be carried out using our recently developed instrument, the scanning micropipette molecule microscope, that will permit the "in vitro" measurement of the transepithelial water permeability of individual cells. This instrument functions as the equivalent of a labelled water-sensitive microelectrode. The results of these experiments will have important consequences for our understanding of the basic mechanisms underlying the normal function of the mammalian kidney. In particular, a demonstration of a role for intercellular communication would be an important link between existing knowledge of the morphology and the biochemistry of hormone response.